Foamable polymers with improved flexibility and low temperature toughness would be desirable for such applications as (1) flexible pipe insulation for high service temperature environments, (2) automotive interior components where soft touch properties are important, e.g., thermoplastic olefin skin/foam systems for instrument panels and/or door panels, and interior roof liners and trunk liners where foam and carpet are laminated and thermoformed to produce a composite structure, and (3) extruded bead applications where impact cushioning properties are desired, e.g. automotive energy absorbers (exterior bumpers, interior pillar cushions and side impact panels), packaging of sensitive electronics, and flotation devices.
Low density polyethylene resins dominate the flexible pipe insulation market at the present time, but lack the thermal stability required for use in industrial and automotive insulation applications.
Propylene-based polymers are attractive because of their high melting point and chemical inertness. High melt strength propylene polymer materials made by irradiating conventional propylene polymers have been used to make foam articles. For example, the use of irradiated, high melt strength propylene polymer materials to make foam articles by expanding the molten polymer with a foaming agent is disclosed in U.S. Pat. No. 4,916,198. Foams made by extruding an irradiated, high melt strength polyolefin composition through a die in the presence of a chlorofluorocarbon blowing agent and a nucleating agent are described in U.S. Pat. No. 5,414,027. A process for making foamed propylene polymer articles by subjecting pre-foamed beads made from an irradiated, high melt strength propylene polymer material to thermoforming by sintering is disclosed in U.S. Pat. No. 5,324,753 and U.S. Pat. No. 5,338,764.
However, the properties of the foamed, irradiated propylene polymer materials made by these processes, particularly toughness, are not adequate at temperatures as low as -20.degree. C.